Method for producing fluorine gas and device for producing fluorine gas

ABSTRACT

A method for producing fluorine gas including electrolyzing an electrolyte in an electrolytic cell, measuring the water concentration in the electrolyte in the electrolyzing, and sending a fluid generated in the inside of the electrolytic cell in the electrolyzing the electrolyte, from the inside to the outside of the electrolytic cell through a flow path. The flow path in which the fluid flows is switched in accordance with the water concentration in the electrolyte measured in the measuring the water concentration, such that the fluid is sent to a first flow path when the water concentration in the electrolyte measured in the measuring the water concentration is not more than a predetermined reference value, or the fluid is sent to a second flow path when the water concentration is more than the predetermined reference value.

TECHNICAL FIELD

The present invention relates to a method for producing fluorine gas anda device for producing fluorine gas.

BACKGROUND ART

Fluorine gas can be synthesized (electrolytically synthesized) byelectrolyzing an electrolyte containing hydrogen fluoride and a metalfluoride. Electrolyzing an electrolyte generates mist (for example, amist of the electrolyte) together with fluorine gas, and thus thefluorine gas sent from an electrolytic cell is accompanied with mist.The mist accompanying fluorine gas becomes fine particles, which mayclog pipes and valves used to send fluorine gas. This may force aproduction operation of fluorine gas to discontinue or stop and hasinterfered with continuous operation to produce fluorine gas by theelectrolytic method.

To suppress clogging of pipes and valves with mist, PTL 1 disclosestechnology of heating fluorine gas accompanied with mist or a pipethrough which the gas passes, to a temperature equal to or higher thanthe melting point of an electrolyte. PTL 2 discloses a gas productiondevice including a gas diffusion unit as a space to roughly collect mistand a filler storage unit storing a filler for adsorbing mist.

CITATION LIST Patent Literature

PTL 1: JP 5584904 B

PTL 2: JP 5919824 B

SUMMARY OF INVENTION Technical Problem

There is still a demand for technology capable of more efficientlysuppress clogging of pipes and valves with mist.

The present invention is intended to provide a method for producingfluorine gas and a device for producing fluorine gas capable ofsuppressing clogging of pipes and valves with mist.

Solution to Problem

To solve the problems, aspects of the present invention are thefollowing [1] to [5].

[1] A method for producing fluorine gas, the fluorine gas being producedby electrolyzing an electrolyte containing hydrogen fluoride and a metalfluoride, the method including

electrolyzing the electrolyte in an electrolytic cell,

measuring a water concentration in the electrolyte in the electrolyzing,and

sending a fluid generated in an inside of the electrolytic cell in theelectrolyzing the electrolyte, from the inside to an outside of theelectrolytic cell through a flow path.

In the method for producing fluorine gas, in the sending, the flow pathin which the fluid flows is switched in accordance with the waterconcentration in the electrolyte measured in the measuring a waterconcentration, such that the fluid is sent to a first flow path thatsends the fluid from the inside of the electrolytic cell to a firstoutside when the water concentration in the electrolyte measured in themeasuring a water concentration is not more than a predeterminedreference value, or the fluid is sent to a second flow path that sendsthe fluid from the inside of the electrolytic cell to a second outsidewhen the water concentration is more than the predetermined referencevalue, and

the predetermined reference value is a numerical value of 0.1% by massor more and 0.8% by mass or less.

[2] The method for producing fluorine gas according to the aspect [1],in which the metal fluoride is a fluoride of at least one metal selectedfrom the group consisting of potassium, cesium, rubidium, and lithium.

[3] The method for producing fluorine gas according to the aspect [1] or[2], in which an anode used in the electrolyzing is a carbonaceouselectrode formed from at least one carbon material selected from thegroup consisting of diamond, diamond-like carbon, amorphous carbon,graphite, and glassy carbon.

[4] The method for producing fluorine gas according to any one of theaspects [1] to [3], in which the electrolytic cell has a structure inwhich bubbles generated on the anode or a cathode used in theelectrolyzing are capable of rising vertically in the electrolyte toreach a surface of the electrolyte.

[5] A device for producing fluorine gas, the fluorine gas being producedby electrolysis of an electrolyte containing hydrogen fluoride and ametal fluoride, the device including

an electrolytic cell storing the electrolyte and configured to performthe electrolysis,

a water concentration measurement unit configured to measure a waterconcentration in the electrolyte in the electrolytic cell during theelectrolysis, and

a flow path configured to send a fluid generated in an inside of theelectrolytic cell during the electrolysis of the electrolyte, from theinside to an outside of the electrolytic cell,

In the device for producing fluorine gas, the flow path includes a firstflow path configured to send the fluid from the inside of theelectrolytic cell to a first outside and a second flow path configuredto send the fluid from the inside of the electrolytic cell to a secondoutside and includes a flow path switching unit configured to switch theflow path in which the fluid flows, to the first flow path or the secondflow path in accordance with the water concentration in the electrolytemeasured by the water concentration measurement unit,

the flow path switching unit is configured to send the fluid from theinside of the electrolytic cell to the first flow path when the waterconcentration in the electrolyte measured by the water concentrationmeasurement unit is not more than a predetermined reference value, or tosend the fluid from the inside of the electrolytic cell to the secondflow path when the water concentration is more than the predeterminedreference value, and

the predetermined reference value is a numerical value of 0.1% by massor more and 0.8% by mass or less.

Advantageous Effects of Invention

According to the present invention, clogging of pipes and valves withmist can be suppressed when an electrolyte containing hydrogen fluorideand a metal fluoride is electrolyzed to produce fluorine gas.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view schematically illustrating an example light scatteringdetector used as an average particle size measurement unit in a devicefor producing fluorine gas pertaining to an embodiment of the presentinvention;

FIG. 2 is a schematic view illustrating an example device for producingfluorine gas pertaining to an embodiment of the present invention;

FIG. 3 is a view schematically illustrating an example mist remover usedas a mist removal unit in the device for producing fluorine gas in FIG.2;

FIG. 4 is a schematic view illustrating a first alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 5 is a schematic view illustrating a second alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 6 is a schematic view illustrating a third alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 7 is a schematic view illustrating a fourth alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 8 is a schematic view illustrating a fifth alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 9 is a schematic view illustrating a sixth alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 10 is a schematic view illustrating a seventh alternativeembodiment of the device for producing fluorine gas in FIG. 2;

FIG. 11 is a schematic view illustrating an eighth alternativeembodiment of the device for producing fluorine gas in FIG. 2;

FIG. 12 is a schematic view illustrating a ninth alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 13 is a schematic view illustrating a tenth alternative embodimentof the device for producing fluorine gas in FIG. 2;

FIG. 14 is a graph illustrating a particle size distribution of a mistcontained in a fluid generated on the anode in Reference Example 1;

FIG. 15 is a graph illustrating a relation between average particle sizeof a mist and amount of the mist generated on the anode in ReferenceExample 1; and

FIG. 16 is a graph illustrating a relation between average particle sizeof a mist and water concentration in an electrolyte in Reference Example1.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described. Theembodiments are merely examples of the present invention, and thepresent invention is not limited to the embodiments. Variousmodifications or improvements can be made in the embodiments, and suchmodifications and improvements can be encompassed by the presentinvention.

The inventors of the present invention have conducted intensive studieson a mist that causes clogging of pipes or valves in electrolyticsynthesis of fluorine gas. In the present invention, a “mist” is liquidmicroparticles or solid microparticles generated together with fluorinegas in an electrolytic cell by electrolysis of an electrolyte.Specifically, a mist is microparticles of an electrolyte, solidmicroparticles formed by phase change of microparticles of anelectrolyte, and solid microparticles generated by reaction of fluorinegas with members included in an electrolytic cell (for example, metalsincluded in an electrolytic cell, gaskets for an electrolytic cell, anda carbon electrode).

The inventors of the present invention have measured the averageparticle size of a mist contained in a fluid generated in anelectrolytic cell during electrolysis of an electrolyte and have foundthat the average particle size of the mist changes with time. As aresult of intensive studies, the inventors of the present invention havealso found a relation between average particle size of a mist and waterconcentration in an electrolyte during electrolysis and have furtherfound a relation between average particle size of a mist and likelihoodof clogging of pipes and valves that send a fluid. The inventors of thepresent invention have found that the clogging of pipes and valves canbe suppressed by improving a flow path for sending a fluid generated inan electrolytic cell in accordance with the water concentration in anelectrolyte during electrolysis, and the frequency of discontinuance orstop of an operation for producing fluorine gas can be reduced and havecompleted the present invention. Embodiments of the present inventionwill now be described.

A method for producing fluorine gas in an embodiment is a method forproducing fluorine gas by electrolyzing an electrolyte containinghydrogen fluoride and a metal fluoride. The method includeselectrolyzing the electrolyte in an electrolytic cell, measuring thewater concentration in the electrolyte in the electrolyzing, and sendinga fluid generated in the inside of the electrolytic cell in theelectrolyzing the electrolyte, from the inside to the outside of theelectrolytic cell through a flow path.

In the sending, the flow path in which the fluid flows is switched inaccordance with the water concentration in the electrolyte measured inthe measuring the water concentration. In other words, the fluid is sentto a first flow path that sends the fluid from the inside of theelectrolytic cell to a first outside when the water concentration in theelectrolyte measured in the measuring the water concentration is notmore than a predetermined reference value, or the fluid is sent to asecond flow path that sends the fluid from the inside of theelectrolytic cell to a second outside when the water concentration ismore than the predetermined reference value. The predetermined referencevalue is a numerical value of 0.1% by mass or more and 0.8% by mass orless.

A device for producing fluorine gas in an embodiment is a device forproducing fluorine gas by electrolysis of an electrolyte containinghydrogen fluoride and a metal fluoride. The device includes anelectrolytic cell storing the electrolyte and configured to perform theelectrolysis, a water concentration measurement unit configured tomeasure the water concentration in the electrolyte in the electrolyticcell during the electrolysis, and a flow path configured to send a fluidgenerated in the inside of the electrolytic cell during the electrolysisof the electrolyte, from the inside to the outside of the electrolyticcell.

The flow path includes a first flow path configured to send the fluidfrom the inside of the electrolytic cell to a first outside and a secondflow path configured to send the fluid from the inside of theelectrolytic cell to a second outside. The flow path also includes aflow path switching unit configured to switch the flow path in which thefluid flows, to the first flow path or the second flow path inaccordance with the water concentration in the electrolyte measured bythe water concentration measurement unit.

The flow path switching unit is configured to send the fluid from theinside of the electrolytic cell to the first flow path when the waterconcentration in the electrolyte measured by the water concentrationmeasurement unit is not more than a predetermined reference value, or tosend the fluid from the inside of the electrolytic cell to the secondflow path when the water concentration is more than the predeterminedreference value. The predetermined reference value is a numerical valueof 0.1% by mass or more and 0.8% by mass or less.

In the method for producing fluorine gas and the device for producingfluorine gas of the embodiments, the flow path in which the fluid flowsis switched to the first flow path or the second flow path in accordancewith the water concentration in the electrolyte during electrolysis. Inother words, the flow path is switched to the first flow path or thesecond flow path in accordance with the average particle size of a mist,and thus the mist is unlikely to cause clogging of the flow paths.Hence, the method for producing fluorine gas and the device forproducing fluorine gas of the embodiments can suppress the clogging ofpipes and valves with mist when an electrolyte containing hydrogenfluoride and a metal fluoride is electrolyzed to produce fluorine gas.This can reduce the frequency of discontinuance or stop of an operationfor producing fluorine gas and facilitates continuous operation. As aresult, fluorine gas can be economically produced.

In the method for producing fluorine gas and the device for producingfluorine gas of the embodiments, the measurement of the waterconcentration in the electrolyte may be performed for an electrolyte inan anode chamber having an anode or for an electrolyte in a cathodechamber having a cathode. The measurement of the water concentration inthe electrolyte may be performed continuously, periodically at regularintervals, or irregularly at any time during electrolysis. The firstflow path differs from the second flow path, but the first outside andthe second outside may be different sections or the same section.

Examples of the method for producing fluorine gas and the device forproducing fluorine gas of the embodiments will be described. The firstflow path is a flow path through which a fluid is sent from the insideof the electrolytic cell through a mist removal unit for removing a mistfrom the fluid to a fluorine gas selection unit for selectivelycollecting fluorine gas from the fluid. The second flow path is a flowpath through which a fluid is sent from the inside of the electrolyticcell to the fluorine gas selection unit but not through the mist removalunit. In other words, a fluid is sent to the mist removal unit on thefirst flow path when the water concentration in the electrolyte is notmore than a predetermined reference value, and a fluid is not sent tothe mist removal unit when the water concentration is more than thepredetermined reference value. In the present example, the fluorine gasselection unit corresponds to the first outside and the second outside,and the first outside and the second outside are the same section, butthe first outside and the second outside may be different sections.

The second flow path has a clogging suppression mechanism thatsuppresses the clogging of the second flow path with mist. The cloggingsuppression mechanism may be any mechanism that can suppress theclogging of the second flow path with mist, and examples include thefollowing mechanisms. In other words, examples include a pipe having alarge diameter, an inclined pipe, a rotary screw, and an airflowgenerator, and these members may be used in combination.

In particular, when the second flow path at least partially includes apipe having a larger diameter than the first flow path, the clogging ofthe second flow path with mist can be suppressed. Alternatively, whenthe second flow path at least partially includes a pipe that is inclinedrelative to the horizontal direction and extends downward from theupstream side to the downstream side, the clogging of the second flowpath with mist can be suppressed.

When a rotary screw for sending a mist deposited in the second flow pathto the upstream side or the downstream side is placed in the second flowpath, the clogging of the second flow path with mist can be suppressed.When the second flow path has an airflow generator for sending airflowto increase the flow rate of a fluid flowing in the second flow path,the clogging of the second flow path with mist can be suppressed.Another mist removal unit different from the mist removal unit on thefirst flow path may be provided on the second flow path as the cloggingsuppression mechanism.

The first flow path is unlikely to be clogged with mist because the mistremoval unit removes a mist from the fluid, and the second flow path isunlikely to be clogged with mist because the clogging suppressionmechanism is provided. Hence, the method for producing fluorine gas andthe device for producing fluorine gas of the embodiments can suppressthe clogging of pipes and valves with mist when an electrolytecontaining hydrogen fluoride and a metal fluoride is electrolyzed toproduce fluorine gas. Even without the mist removal unit or the cloggingsuppression mechanism, simply switching the flow path in which a fluidflows, to another flow path (a first flow path or a second flow path)can achieve the effect of suppressing clogging of pipes and valves withmist, but the effect is highly achieved when the mist removal unit orthe clogging suppression mechanism is provided.

The method for producing fluorine gas and the device for producingfluorine gas of the embodiments will next be described in furtherdetail.

[Electrolytic Cell]

The electrolytic cell may be any cell that can electrolyze anelectrolyte containing hydrogen fluoride and a metal fluoride togenerate fluorine gas.

Typically, the inside of the electrolytic cell is sectioned by apartition member such as a partition wall into an anode chamber havingan anode and a cathode chamber having a cathode, and this structureprevents the fluorine gas generated on the anode from mixing with thehydrogen gas generated on the cathode.

As the anode, for example, a carbonaceous electrode formed from a carbonmaterial such as diamond, diamond-like carbon, amorphous carbon,graphite, glassy carbon, and indefinite carbon can be used. As theanode, a metal electrode formed from a metal such as nickel and Monel(trademark) can also be used in addition to the carbon material. As thecathode, for example, a metal electrode formed from a metal such asiron, copper, nickel, and Monel (trademark) can be used.

The electrolyte contains hydrogen fluoride and a metal fluoride. Themetal fluoride may be any type and is preferably a fluoride of at leastone metal selected from the group consisting of potassium, cesium,rubidium, and lithium. When containing cesium or rubidium, theelectrolyte has a larger specific gravity and thus suppresses the amountof a mist generated during electrolysis.

As the electrolyte, for example, a mixed molten salt of hydrogenfluoride (HF) and potassium fluoride (KF) can be used. In the mixedmolten salt of hydrogen fluoride and potassium fluoride, the molar ratioof hydrogen fluoride to potassium fluoride can be, for example, hydrogenfluoride:potassium fluoride=1.5 to 2.5:1. A typical electrolyte isKF.2HF where the ratio of hydrogen fluoride to potassium fluoride is2:1, and the mixed molten salt has a melting point of about 72° C. Theelectrolyte has corrosivity, and thus a portion to come into contactwith the electrolyte, such as the inner face of the electrolytic cell,is preferably formed from a metal such as iron, nickel, and Monel(trademark).

During electrolysis of the electrolyte, a direct current is applied tothe anode and the cathode. Accordingly, a gas containing fluorine gas isgenerated on the anode, whereas a gas containing hydrogen gas isgenerated on the cathode. The hydrogen fluoride in the electrolyte has avapor pressure, and thus gases generated on the anode and the cathodeare accompanied with hydrogen fluoride. In the production of fluorinegas by electrolysis of an electrolyte, a gas generated by theelectrolysis also contains a mist of the electrolyte. Accordingly, thegas phase in the electrolytic cell contains a gas generated byelectrolysis, hydrogen fluoride, and a mist of the electrolyte. Hence,the substance sent from the inside to the outside of the electrolyticcell contains a gas generated by electrolysis, hydrogen fluoride, and amist of the electrolyte and is called a “fluid” in the presentinvention.

As electrolysis proceeds, the hydrogen fluoride in the electrolyte isconsumed, and thus a pipe for continuously or intermittently feeding andresupplying hydrogen fluoride into the electrolytic cell may beconnected to the electrolytic cell. Hydrogen fluoride may be fed toeither the cathode chamber or the anode chamber of the electrolyticcell.

A mist is generated during electrolysis of an electrolyte mainly due tothe following reason. The temperature of an electrolyte duringelectrolysis is adjusted, for example, at 80 to 100° C. KF.2HF has amelting point of 71.7° C., and thus the electrolyte is in the liquidstate when the temperature is adjusted as above. Bubbles of the gasgenerated on both the electrodes in the electrolytic cell rise in theelectrolyte and burst on the surface of the electrolyte. On thebursting, the electrolyte is partially discharged into the gas phase.

The gas phase has a temperature lower than the melting point of theelectrolyte, and thus the discharged electrolyte changes in phase intosuch a state as microscopic particles. The fine particles are supposedlya mixture of potassium fluoride and hydrogen fluoride, KF.nHF. The fineparticles float on a separately generated gas and become a mist, forminga fluid generated in the electrolytic cell. Such a mist has tackinessand the like and thus is difficult to efficiently remove by conventionalcountermeasures such as installation of filters.

A carbonaceous electrode as the anode may be reacted with fluorine gasgenerated by electrolysis to generate impalpable particles of an organiccompound as a mist in a small amount. Specifically, an electric currentsupply portion to the carbonaceous electrode has a contact resistance inmany cases and may have a temperature higher than the temperature of theelectrolyte due to Joule heat. Hence, the carbon included in thecarbonaceous electrode may be reacted with fluorine gas to generate asoot-like organic compound, CFx, as a mist.

The electrolytic cell preferably has a structure in which bubblesgenerated on the anode or the cathode used in the electrolysis canvertically rise in the electrolyte to reach the surface of theelectrolyte. In an electrolytic cell having a structure in which bubblesare unlikely to vertically rise but rise in a direction inclinedrelative to the vertical direction, a plurality of bubbles are likely togather to form large bubbles. The resulting large bubbles reach thesurface of the electrolyte and burst, and the amount of a mist is likelyto increase. When an electrolytic cell has a structure in which bubblescan vertically rise in an electrolyte to reach the surface of theelectrolyte, small bubbles reach the surface of the electrolyte andburst, and thus the amount of a mist is likely to decrease.

[Average Particle Size Measurement Unit]

The device for producing fluorine gas of the embodiment may have anaverage particle size measurement unit for measuring the averageparticle size of a mist contained in a fluid. The average particle sizemeasurement unit may include a light scattering detector for measuringthe average particle size by light scattering. The light scatteringdetector can measure the average particle size of a mist in a fluidflowing in a flow path while the device for producing fluorine gas iscontinuously operated and thus is preferred as the average particle sizemeasurement unit.

An example light scattering detector will be described with reference toFIG. 1. The light scattering detector in FIG. 1 is a light scatteringdetector usable as the average particle size measurement unit in thedevice for producing fluorine gas of the embodiment (for example, thedevices for producing fluorine gas in FIG. 2 and FIGS. 4 to 13 describedlater). In other words, the light scattering detector measures theaverage particle size of a mist contained in a fluid generated in theelectrolytic cell when an electrolyte containing hydrogen fluoride and ametal fluoride is electrolyzed in the electrolytic cell of the devicefor producing fluorine gas to produce fluorine gas.

The light scattering detector may be connected to the device forproducing fluorine gas, and the average particle size of a mist may bemeasured while a fluid is sent from the inside of the electrolytic cellto the light scattering detector. Alternatively, the light scatteringdetector may not be connected to the device for producing fluorine gasand may measure the average particle size of a mist while a fluid issampled from the inside of the electrolytic cell and is introduced tothe light scattering detector.

The light scattering detector in FIG. 1 includes a sample chamber 1 forreceiving a fluid F, a light source 2 for applying light for lightscattering measurement L to the fluid F in the sample chamber 1, ascattered light detection unit 3 for detecting scattered light Sgenerated when the light for light scattering measurement L is scatteredby a mist M in the fluid F, a transparent window 4A that is placed inthe sample chamber 1 and is in contact with the fluid F and throughwhich the light for light scattering measurement L passes, and atransparent window 4B that is placed in the sample chamber 1 and is incontact with the fluid F and through which the scattered light S passes.The transparent windows 4A, 4B are formed from at least one selectedfrom the group consisting of diamond, calcium fluoride (CaF₂), potassiumfluoride (KF), silver fluoride (AgF), barium fluoride (BaF₂), andpotassium bromide (KBr).

The light for light scattering measurement L (for example, a laser beam)emitted from the light source 2 passes through a converging lens 6 andthe transparent window 4A of the sample chamber 1, enters the samplechamber 1, and is applied to the fluid F received in the sample chamber1. At the application, when the fluid F contains a light reflectivesubstance such as a mist M, the light for light scattering measurement Lis reflected and scattered. The scattered light S generated when thelight for light scattering measurement L is scattered by the mist Mpartially passes through the transparent window 4B of the sample chamber1, is retrieved from the sample chamber 1 to the outside, and enters thescattered light detection unit 3 through a condensing lens 7 and athrottle 8. From the information of the scattered light S, the averageparticle size of the mist M can be determined. The average particle sizedetermined by the detector is a number average particle size. As thescattered light detection unit 3, for example, an aerosol spectrometer,Welas (registered trademark) digital 2000 manufactured by PALAS can beused.

The transparent windows 4A, 4B are in contact with the fluid F. Thefluid F contains highly reactive fluorine gas, and thus the transparentwindows 4A, 4B are required to be formed from a material that isunlikely to be corroded by fluorine gas. The material for forming thetransparent windows 4A, 4B is, for example, at least one selected fromthe group consisting of diamond, calcium fluoride, potassium fluoride,silver fluoride, barium fluoride, and potassium bromide. When thetransparent windows 4A, 4B are formed from such a material as above, thedeterioration by contact with the fluid F can be suppressed.

A glass such as quartz having a surface coated with a film formed ofsuch a material as above can also be used as the transparent windows 4A,4B. The portion to come into contact with the fluid F is coated with afilm formed of such a material as above, and thus the deterioration bycontact with the fluid F can be suppressed while the cost is reduced.Each transparent window 4A, 4B may be a laminate in which a face to comeinto contact with the fluid F is formed of such a material as above, andthe other portions are formed of a common glass such as quartz.

The members of the light scattering detector except the transparentwindows 4A, 4B may be made from any material having corrosion resistanceagainst fluorine gas, and, for example, a metal material such as Monel(trademark) that is a copper-nickel alloy, hastelloy (trademark), andstainless steel is preferably used.

[Average Particle Size of Mist and Water Concentration in Electrolyte]

The inventors of the present invention measured the average particlesize of a mist generated during production of fluorine gas byelectrolysis of an electrolyte, by using the light scattering detector.An example of the result will be described. After the anode of a devicefor producing fluorine gas is exchanged for a new anode or anelectrolytic cell is filled with a fresh electrolyte, electrolysis isstarted, and the average particle size of a mist in a fluid generated onthe anode was measured for a certain period of time from just after thestart of electrolysis. As a result, the mist had an average particlesize of 0.5 to 2.0 μm. After a sufficient time period of continuouselectrolysis, the electrolysis is becoming stable. During the stableelectrolysis, the mist in the fluid had an average particle size ofabout 0.2 μm.

As described above, a mist having a relatively large particle size isgenerated from just after the start of electrolysis to the stableelectrolysis. If the fluid containing a mist having a large size justafter the start of electrolysis flows in pipes and valves, the mist islikely to adsorb onto the inner face of the pipes and valves, causingclogging of the pipes and valves.

In contrast, during stable electrolysis, the generated mist has arelatively small particle size. Such a small mist is unlikely to settleor deposit in a fluid and thus can flow stably in pipes and valves.Hence, during stable electrolysis, a fluid consisting of a mist and agas generated on an electrode has a relatively low possibility ofcausing clogging of pipes and valves. The time from the start ofelectrolysis to the stable electrolysis is typically 25 hours or moreand 200 hours or less. From the start of electrolysis to the stableelectrolysis, an electric energy of about 40 kAh or more is required tobe applied for 1,000 L of an electrolyte.

The inventors of the present invention have found a close relationbetween the average particle size of a mist and the water concentrationin an electrolyte. The water concentration in an electrolyte istypically large at the start of electrolysis and is larger than 1.0% bymass. At the start of electrolysis, the mist has an average particlesize of more than 0.4 μm. As the electrolysis is continued, theelectrolyte has a lower water concentration, and when the waterconcentration reaches 0.3% by mass or less, the mist has an averageparticle size of 0.4 μm or less.

As described above, the average particle size of a mist has a relationto the water concentration in an electrolyte. Hence, the waterconcentration in an electrolyte can be measured during electrolysis inplace of the average particle size of a mist, and the measurement resultcan be used to switch a flow path. In other words, when the waterconcentration in an electrolyte is measured at predetermined timingduring electrolysis, the flow path in which a fluid generated by theelectrolysis flows can be appropriately switched at the predeterminedtiming in accordance with the measurement result.

The water concentration in an electrolyte decreases depending on themagnitude of current value or the electric energy (product of currentvalue and electrolysis time) . As the current value is larger, the waterconcentration decreases rapidly. When a carbonaceous electrode thatcauses an anode effect of rapidly increasing the electrical voltage ofan anode is used as the anode, electrolysis is performed at an anodecurrent density of less than 0.1 A/cm². The water concentration may bereduced at a constant current density, or the water concentration may bereduced while the current density is gradually increased.

Based on such knowledge, the inventors of the present invention haveinvented the method for producing fluorine gas and the device forproducing fluorine gas, having a structure in which a flow path in whicha fluid flows can be switched in accordance with the water concentrationin an electrolyte during electrolysis. The device for producing fluorinegas of the embodiment has a first flow path and a second flow path, anda flow path switching unit (for example, a switching valve) maybe usedto select, from the two flow paths, a flow path used to convey a fluid.

Alternatively, the device for producing fluorine gas of the embodimentmay have two flow paths and a transfer and replacement mechanism fortransferring and replacing an electrolytic cell. From the two flowpaths, a flow path used to convey a fluid may be selected, and anelectrolytic cell maybe transferred near the flow path and be connectedto the flow path. This can switch the flow path.

The device has the first flow path and the second flow path as describedabove. Hence, even while one flow path is blocked and cleaned, the otherflow path can be opened, and the device for producing fluorine gas canbe continuously operated.

From studies by the inventors of the present invention, a mist having arelatively large average particle size is generated from the start ofelectrolysis to the stable electrolysis, and thus a fluid can be sent tothe second flow path having a clogging suppression mechanism. When theelectrolysis becomes stable as time passes, a mist having a relativelysmall average particle size is generated, and thus the flow path can beswitched such that the fluid is sent to the first flow path having amist removal unit.

Such switching the flow path is performed in accordance with themeasured water concentration in the electrolyte, and the flow path isswitched on the basis of a predetermined reference value. Theappropriate reference value of the average particle size of a mistgenerated on an anode varies with devices and is, for example, 0.1 μm ormore and 1.0 μm or less, preferably 0.2 μm or more and 0.8 μm or less,and more preferably 0.4 μm.

From the relation between the average particle size of a mist and thewater concentration in an electrolyte, the appropriate reference valueof the water concentration in an electrolyte is accordingly 0.1% by massor more and 0.8% by mass or less, preferably 0.2% by mass or more and0.6% by mass or less, and more preferably 0.3% by mass. When the waterconcentration in an electrolyte is more than a reference value, thefluid can be sent to the second flow path, and when the waterconcentration is not more than the reference value, the fluid can besent to the first flow path.

The water concentration in an electrolyte can be determined, forexample, by Karl Fischer method. Alternatively, the water concentrationin an electrolyte can also be determined by heating the electrolyte, forexample, at 250° C. or more and 400° C. or less and determining thewater amount in the generated gas by infrared spectroscopy. A solidelectrolyte is hardly dissolved in a detection liquid used for the KarlFischer method, and thus another solvent is required to dissolve thesolid electrolyte, but almost no solvent has a large solubility of asolid electrolyte. Accordingly, it is difficult to dissolve a largeamount of a solid electrolyte for the Karl Fischer analysis, and thusthe Karl Fischer method is suitable for analysis of a solid electrolytehaving a high water content. In contrast, a method of heating a solidelectrolyte and measuring the amount of water in the generated gasinvolves a longer analysis time than the Karl Fischer method but canaccurately analyze the water concentration in an electrolyte.

A fluid (mainly containing hydrogen gas) generated on the cathode, forexample, contains 20 to 50 μg of fine particles (calculated assumingthat a mist has a specific gravity of 1.0 g/mL) per unit volume (1liter), and the fine particles have an average particle size of about0.1 μm with a distribution of ±0.05 μm.

In the fluid generated on the cathode, a large difference in particlesize distribution of the generated fine particles was not observed evenwhen the water concentration in an electrolyte varied. The mistcontained in the fluid generated on the cathode has a smaller averageparticle size than the mist contained in the fluid generated on theanode and thus is unlikely to cause clogging of pipes and valves ascompared with the mist contained in the fluid generated on the anode.Hence, the mist contained in the fluid generated on the cathode can beremoved from the fluid by using an appropriate removal method.

An example of the device for producing fluorine gas of the embodimentwill be described in detail with reference to FIG. 2. The device forproducing fluorine gas in FIG. 2 is an example including twoelectrolytic cells, but a single electrolytic cell may be included, orthree or more, for example, 10 to 15 electrolytic cells may be included.

The device for producing fluorine gas illustrated in FIG. 2 includeselectrolytic cells 11, 11 in which an electrolyte 10 is stored andelectrolysis is performed, an anode 13 placed in each electrolytic cell11 and immersed in the electrolyte 10, and a cathode 15 placed in eachelectrolytic cell 11, immersed in the electrolyte 10, and facing theanode 13.

The inside of each electrolytic cell 11 is sectioned into an anodechamber 22 and a cathode chamber 24 by a partition wall 17 extendingfrom a ceiling face in the electrolytic cell 11 downward in the verticaldirection and having a lower end immersed in the electrolyte 10. In theanode chamber 22, the anode 13 is placed, and in the cathode chamber 24,the cathode 15 is placed. The space above the surface of the electrolyte10 is separated by the partition wall 17 into a space in the anodechamber 22 and a space in the cathode chamber 24, and a portion of theelectrolyte 10 above the lower end of the partition wall 17 is separatedby the partition wall 17, but a portion of the electrolyte 10 below thelower end of the partition wall 17 is not directly separated by thepartition wall 17 but continues.

The device for producing fluorine gas illustrated in FIG. 2 includeswater concentration measurement units 36 that measure the waterconcentration in the electrolyte 10 in the electrolytic cells 11 duringelectrolysis of the electrolyte 10, a first average particle sizemeasurement unit 31 that measures the average particle size of a mistcontained in a fluid generated in each electrolytic cell 11 duringelectrolysis of the electrolyte 10, a first mist removal unit 32 thatremoves a mist from a fluid, a fluorine gas selection unit (notillustrated) that selectively collects fluorine gas from a fluid, and aflow path configured to send a fluid from the inside of eachelectrolytic cell 11 to the fluorine gas selection unit.

The flow path includes a first flow path that sends a fluid from theinside of each electrolytic cell 11 through the first mist removal unit32 to the fluorine gas selection unit and a second flow path that sendsa fluid from the inside of each electrolytic cell 11 to the fluorine gasselection unit but not through the first mist removal unit 32. The flowpath has a flow path switching unit that switches the flow path in whicha fluid flows, to the first flow path or the second flow path inaccordance with the water concentration in the electrolyte 10 measuredby the water concentration measurement unit 36. In other words, at anintermediate point of the flow path extending from the electrolytic cell11, the flow path switching unit is provided, and the flow pathswitching unit can alter the flow path in which a fluid flows.

The flow path switching unit sends a fluid from the inside of eachelectrolytic cell 11 to the first flow path when the water concentrationin the electrolyte 10 measured by the water concentration measurementunit 36 is not more than a predetermined reference value or sends afluid from the inside of each electrolytic cell 11 to the second flowpath when the water concentration is more than the predeterminedreference value. The second flow path has a clogging suppressionmechanism that suppresses the clogging of the second flow path withmist.

In other words, when the water concentration in the electrolyte 10 isnot more than a reference value, the electrolytic cell 11 is connectedto a fluorine gas selection unit, and the fluid is sent to the firstflow path with the first mist removal unit 32. When the waterconcentration in the electrolyte 10 is more than the reference value,the electrolytic cell 11 is connected to a fluorine gas selection unit,and the fluid is sent to the second flow path with the cloggingsuppression mechanism.

As the water concentration measurement unit 36, for example, a KarlFischer moisture meter can be used.

As the first mist removal unit 32, for example, a mist remover capableof removing a mist having an average particle size of 0.4 μm or lessfrom a fluid is used. The type of mist remover, or the system ofremoving a mist is not specifically limited, but a mist has a smallaverage particle size, and thus, for example, an electric dustcollector, a venturi scrubber, or a filter can be used as the mistremover.

Of the above mist removers, the mist remover illustrated in FIG. 3 ispreferably used. The mist remover illustrated in FIG. 3 is a scrubbertype mist remover using a liquid hydrogen fluoride as a circulatingliquid. The mist remover illustrated in FIG. 3 can efficiently remove amist having an average particle size of 0.4 μm or less from a fluid. Themist remover uses a liquid hydrogen fluoride as a circulating liquid.The circulating liquid is preferably cooled in order to reduce theconcentration of hydrogen fluoride in a fluorine gas, and thus theconcentration of hydrogen fluoride in a fluorine gas can be controlledby adjusting the cooling temperature.

The device for producing fluorine gas illustrated in FIG. 2 will bedescribed in further detail. A first pipe 41 that sends a fluidgenerated in the anode chamber 22 in each electrolytic cell 11(hereinafter also called “anode gas”) to the outside connects theelectrolytic cell 11 to a fourth pipe 44, and the anode gases sent fromthe two electrolytic cells 11, 11 are sent through the first pipes 41 tothe fourth pipe 44 and are mixed. The main component of the anode gas isfluorine gas, and accessory components are mist, hydrogen fluoride,carbon tetrafluoride, oxygen gas, and water.

The fourth pipe 44 is connected to the first mist removal unit 32, andthe anode gas is sent through the fourth pipe 44 to the first mistremoval unit 32. The first mist removal unit 32 removes mist andhydrogen fluoride in the anode gas from the anode gas. The anode gasfrom which the mist and hydrogen fluoride have been removed is sent fromthe first mist removal unit 32 through a sixth pipe 46 connected to thefirst mist removal unit 32 to a fluorine gas selection unit (notillustrated). The fluorine gas selection unit then selectively collectsfluorine gas from the anode gas.

The first mist removal unit 32 is connected to an eighth pipe 48, and aliquid hydrogen fluoride as the circulating liquid is supplied throughthe eighth pipe 48 to the first mist removal unit 32. The first mistremoval unit 32 is further connected to a ninth pipe 49. The ninth pipe49 is connected through third pipes 43 to the electrolytic cells 11, 11,and a circulating liquid (liquid hydrogen fluoride) containing a mistand having used to remove a mist in the first mist removal unit 32 isreturned from the first mist removal unit 32 to the electrolytic cells11, 11.

The cathode chamber 24 in each electrolytic cell 11 is substantially thesame as the anode chamber 22. In other words, a second pipe 42 thatsends a fluid generated in the cathode chamber 24 in each electrolyticcell 11 (hereinafter also called “cathode gas”) to the outside connectsthe electrolytic cell 11 to a fifth pipe 45, and the cathode gases sentfrom the two electrolytic cells 11, 11 are sent through the second pipes42 to the fifth pipe 45 and are mixed. The main component of the cathodegas is hydrogen gas, and accessory components are mist, hydrogenfluoride, and water.

The cathode gas contains a fine mist and 5 to 10% by volume of hydrogenfluoride, and thus it is unfavorable to directly discharge the cathodegas to the atmosphere. To address this, the fifth pipe 45 is connectedto a second mist removal unit 33, and the cathode gas is sent throughthe fifth pipe 45 to the second mist removal unit 33. The second mistremoval unit 33 removes mist and hydrogen fluoride in the cathode gasfrom the cathode gas. The cathode gas from which the mist and hydrogenfluoride have been removed is discharged from the second mist removalunit 33 through a seventh pipe 47 connected to the second mist removalunit 33 to the atmosphere. The type of second mist removal unit 33, orthe system of removing a mist is not specifically limited, and ascrubber type mist remover using an aqueous alkali solution as thecirculating liquid can be used.

The pipe diameters and the installation directions (i.e., a pipeextending direction, for example, the vertical direction, the horizontaldirection) of the first pipe 41, the second pipe 42, the fourth pipe 44,and the fifth pipe 45 are not specifically limited. The first pipe 41and the second pipe 42 are preferably installed so as to extend from theelectrolytic cell 11 in the vertical direction and preferably have apipe diameter such that fluids flowing in the first pipe 41 and thesecond pipe 42 have a flow rate of 30 cm/sec or less in a normal state.In such conditions, even when a mist contained in a fluid falls underits own weight, the mist settles in the electrolytic cell 11, and thusthe clogging in the first pipe 41 and the second pipe 42 with fineparticles is unlikely to be caused.

The fourth pipe 44 and the fifth pipe 45 are preferably installed so asto extend in the horizontal direction and preferably have a pipediameter such that fluids flowing in the fourth pipe 44 and the fifthpipe 45 have a flow rate about 1 to 10 times more than that in the firstpipe 41 and the second pipe 42.

A second bypass pipe 52 for sending the anode gas to the outside of theelectrolytic cell 11 is further provided separately from the first pipe41. In other words, the second bypass pipe 52 connects each electrolyticcell 11 to a first bypass pipe 51, and the anode gases sent from the twoelectrolytic cells 11, 11 are sent through the second bypass pipes 52 tothe first bypass pipe 51 and are mixed. Through the first bypass pipe51, the anode gas is sent to a fluorine gas selection unit (notillustrated). The fluorine gas selection unit selectively collectsfluorine gas from the anode gas. The fluorine gas selection unitconnected to the first bypass pipe 51 may be the same as or differentfrom the fluorine gas selection unit connected to the sixth pipe 46.

The pipe diameter and the installation direction of the second bypasspipe 52 are not specifically limited, and the second bypass pipe 52 ispreferably installed so as to extend from the electrolytic cell 11 inthe vertical direction and preferably has a pipe diameter such that afluid flowing in the second bypass pipe 52 has a flow rate of 30 cm/secor less in a normal state.

The first bypass pipe 51 is installed so as to extend in the horizontaldirection. The first bypass pipe 51 has a larger pipe diameter than thefourth pipe 44, and the pipe diameter of the first bypass pipe 51 issuch a size as to be unlikely to cause clogging of the first bypass pipe51 with depositing fine particles. The first bypass pipe 51 has a largerpipe diameter than the fourth pipe 44, and this functions as theclogging suppression mechanism.

The pipe diameter of the first bypass pipe 51 is preferably more than1.0 time and not more than 3.2 times that of the fourth pipe 44 and morepreferably not less than 1.05 times and not more than 1.5 times. Inother words, the first bypass pipe 51 preferably has a flow pathcross-sectional area not more than 10 times that of the fourth pipe 44.

As apparent from the above description, the first pipes 41 and thefourth pipe 44 constitute the above first flow path, and the firstbypass pipe 51 and the second bypass pipes 52 constitute the abovesecond flow path. The first bypass pipe 51 included in the second flowpath has the clogging suppression mechanism.

Next, the flow path switching unit will be described. Each first pipe 41has a first pipe valve 61. By switching the first pipe valve 61 to anopen state or a closed state, whether the anode gas is sent from theelectrolytic cell 11 to the first mist removal unit 32 can becontrolled. Each second bypass pipe 52 has a bypass valve 62. Byswitching the bypass valve 62 to an open state or a closed state,whether the anode gas is sent from the electrolytic cell 11 to the firstbypass pipe 51 can be controlled.

On the electrolytic cell 11, a water concentration measurement unit 36is provided, and the water concentration in the electrolyte 10 can bemeasured during electrolysis by introducing the electrolyte 10 in theelectrolytic cell 11 to the water concentration measurement unit 36. Theelectrolyte 10 for measuring the water concentration may be either anelectrolyte 10 in the anode chamber 22 or an electrolyte 10 in thecathode chamber 24.

Between the electrolytic cells 11 and the first mist removal unit 32,specifically, at an intermediate point of the fourth pipe 44 and at thedownstream side of the junctions to the first pipes 41, a first averageparticle size measurement unit 31 is provided. The first averageparticle size measurement unit 31 measures the average particle size ofa mist contained in the anode gas flowing in the fourth pipe 44. Byanalyzing fluorine gas and nitrogen gas contained in the anode gas aftermeasuring the average particle size of a mist, the current efficiency inthe production of fluorine gas can be determined.

At an intermediate point of the first bypass pipe 51 and at thedownstream side of the junctions to the second bypass pipes 52, a secondaverage particle size measurement unit 34 is also provided, and thesecond average particle size measurement unit 34 measures the averageparticle size of a mist contained in the anode gas flowing in the firstbypass pipe 51. The device for producing fluorine gas illustrated inFIG. 2 may not include the first average particle size measurement unit31 or the second average particle size measurement unit 34.

The water concentration in the electrolyte 10 in the electrolytic cell11 is measured by the water concentration measurement unit 36. When themeasurement result is more than a predetermined reference value, thebypass valve 62 is switched to an open state to send the anode gas fromthe electrolytic cell 11 to the first bypass pipe 51, and the first pipevalve 61 is switched to a closed state not to send the anode gas to thefourth pipe 44 and the first mist removal unit 32. In other words, theanode gas is sent to the second flow path.

In contrast, when the measurement result is not more than apredetermined reference value, the first pipe valve 61 is switched to anopen state to send the anode gas to the fourth pipe 44 and the firstmist removal unit 32, and the bypass valve 62 is switched to a closedstate not to send the anode gas from the electrolytic cell 11 to thefirst bypass pipe 51. In other words, the anode gas is sent to the firstflow path.

As apparent from the above description, the first pipe valve 61 and thebypass valve 62 constitute the above flow path switching unit.

As described above, by operating the device for producing fluorine gaswhile the flow path is switched in accordance with the waterconcentration in the electrolyte 10 during electrolysis, continuousoperation can be smoothly performed while clogging of pipes and valveswith mist is suppressed. By using the device for producing fluorine gasillustrated in FIG. 2, fluorine gas can be economically produced.

For example, as the mist removal unit, a plurality of pipes with filtersmay be prepared, and electrolysis may be performed while the pipes areappropriately switched to exchange the filters.

A time period when frequent exchange of filters is needed and a timeperiod when frequent exchange of filters is not needed can be determinedby measuring the water concentration in the electrolyte 10 duringelectrolysis. By appropriately controlling the switching frequency ofpipes in which a fluid flows on the basis of the above determination,the device for producing fluorine gas can be efficiently, continuouslyoperated.

Alternative embodiments of the device for producing fluorine gasillustrated in FIG. 2 will next be described.

First Alternative Embodiment

A first alternative embodiment will be described with reference to FIG.4. In the device for producing fluorine gas illustrated in FIG. 2, thesecond bypass pipes 52 connect the electrolytic cells 11 to the firstbypass pipe 51. In contrast, in a device for producing fluorine gas inthe first alternative embodiment illustrated in FIG. 4, second bypasspipes 52 connect first pipes 41 to a first bypass pipe 51. The devicefor producing fluorine gas in the first alternative embodiment hassubstantially the same constitution as the device for producing fluorinegas in FIG. 2 except the above structure, and thus similar structuresare not described.

Second Alternative Embodiment

A second alternative embodiment will be described with reference to FIG.5. A device for producing fluorine gas in the second alternativeembodiment illustrated in FIG. 5 includes a single electrolytic cell 11.A first average particle size measurement unit 31 is not provided on afourth pipe 44 but on a first pipe 41 and is provided at the upstreamside of a first pipe valve 61. The device includes no second bypass pipe52, and a first bypass pipe 51 is directly connected to an electrolyticcell 11 but not through a second bypass pipe 52.

The first bypass pipe 51 has a larger diameter than the fourth pipe 44and thus functions as the clogging suppression mechanism. A mist poolspace is further provided, for example, at the downstream end of thefirst bypass pipe 51, and this can further improve the cloggingsuppression effect. Examples of the mist pool space include a spaceformed from the downstream end portion of the first bypass pipe 51 andhaving a larger pipe diameter than the center portion in theinstallation direction (for example, a pipe diameter not less than 4times that at the center portion in the installation direction) and aspace formed from the downstream end portion of the first bypass pipe 51and having a container shape. The mist pool space can suppress cloggingof the first bypass pipe 51. This is aimed at a clogging suppressioneffect by a large flow path cross-sectional area and a cloggingsuppression effect using mist free fall by a reduction in linearvelocity of a flowing gas.

In addition, a bypass valve 62 is provided on a third bypass pipe 53that connects the first bypass pipe 51 to a fluorine gas selection unit(not illustrated). The device for producing fluorine gas in the secondalternative embodiment has substantially the same constitution as thedevice for producing fluorine gas in FIG. 2 except the above structure,and thus similar structures are not described.

Third Alternative Embodiment

A third alternative embodiment will be described with reference to FIG.6. In a device for producing fluorine gas in the third alternativeembodiment, a first average particle size measurement unit 31 isprovided on an electrolytic cell 11, and the average particle size of amist is measured by introducing the anode gas in the electrolytic cell11 directly into the first average particle size measurement unit 31.The device for producing fluorine gas in the third alternativeembodiment has no second average particle size measurement unit 34. Thedevice for producing fluorine gas in the third alternative embodimenthas substantially the same constitution as the device for producingfluorine gas in the second alternative embodiment except the abovestructure, and thus similar structures are not described.

Fourth Alternative Embodiment

A fourth alternative embodiment will be described with reference to FIG.7. A device for producing fluorine gas in the fourth alternativeembodiment differs from that in the second alternative embodimentillustrated in FIG. 5 in the clogging suppression mechanism. In thedevice for producing fluorine gas in the second alternative embodiment,the first bypass pipe 51 is provided so as to extend in the horizontaldirection. In the device for producing fluorine gas in the fourthalternative embodiment, a first bypass pipe 51 is inclined relative tothe horizontal direction and extends downward from the upstream side tothe downstream side. This inclination prevents fine particles fromdepositing in the first bypass pipe 51. As the inclination is larger,the effect of suppressing fine particle deposition is larger.

The inclination angle of the first bypass pipe 51 is preferably 30degrees or more and more preferably 40 degrees or more and 60 degrees orless where the depression angle from the horizontal plane is less than90 degrees. When the first bypass pipe 51 is about to be clogged,hammering the inclined first bypass pipe 51 facilitates moving thedeposit in the first bypass pipe 51, and thus clogging can be prevented.

The device for producing fluorine gas in the fourth alternativeembodiment has substantially the same constitution as the device forproducing fluorine gas in the second alternative embodiment except theabove structure, and thus similar structures are not described.

Fifth Alternative Embodiment

A fifth alternative embodiment will be described with reference to FIG.8. A device for producing fluorine gas in the fifth alternativeembodiment differs from that in the third alternative embodimentillustrated in FIG. 6 in the clogging suppression mechanism. In thedevice for producing fluorine gas in the third alternative embodiment,the first bypass pipe 51 is provided so as to extend in the horizontaldirection. In the device for producing fluorine gas in the fifthalternative embodiment, a first bypass pipe 51 is inclined relative tothe horizontal direction and extends downward from the upstream side tothe downstream side. This inclination prevents fine particles fromdepositing in the first bypass pipe 51. The inclination angle of thefirst bypass pipe 51 is preferably substantially the same as in thefourth alternative embodiment. The device for producing fluorine gas inthe fifth alternative embodiment has substantially the same constitutionas the device for producing fluorine gas in the third alternativeembodiment except the above structure, and thus similar structures arenot described.

Sixth Alternative Embodiment

A sixth alternative embodiment will be described with reference to FIG.9. A device for producing fluorine gas in the sixth alternativeembodiment differs from that in the second alternative embodimentillustrated in FIG. 5 in the structure of an electrolytic cell 11. Theelectrolytic cell 11 has one anode 13 and two cathodes 15, 15 and issectioned into one anode chamber 22 and one cathode chamber 24 by acylindrical partition wall 17 surrounding the one anode 13. The anodechamber 22 is formed to extend above the top face of the electrolyticcell 11, and a first bypass pipe 51 is connected to the top section ofthe anode chamber 22 of the electrolytic cell 11. The device forproducing fluorine gas in the sixth alternative embodiment hassubstantially the same constitution as the device for producing fluorinegas in the second alternative embodiment except the above structure, andthus similar structures are not described.

Seventh Alternative Embodiment

A seventh alternative embodiment will be described with reference toFIG. 10. A device for producing fluorine gas in the seventh alternativeembodiment differs from that in the sixth alternative embodimentillustrated in FIG. 9 in the structure of a first bypass pipe 51. Inother words, in the device for producing fluorine gas in the seventhalternative embodiment, a first bypass pipe 51 is inclined relative tothe horizontal direction and extends downward from the upstream side tothe downstream side as with the fourth alternative embodiment and thefifth alternative embodiment. The inclination angle of the first bypasspipe 51 is preferably substantially the same as in the fourthalternative embodiment. The device for producing fluorine gas in theseventh alternative embodiment has substantially the same constitutionas the device for producing fluorine gas in the sixth alternativeembodiment except the above structure, and thus similar structures arenot described.

Eighth Alternative Embodiment

An eighth alternative embodiment will be described with reference toFIG. 11. A device for producing fluorine gas in the eighth alternativeembodiment differs from that in the second alternative embodimentillustrated in FIG. 5 in the clogging suppression mechanism. In thedevice for producing fluorine gas in the eighth alternative embodiment,a rotary screw 71 constituting the clogging suppression mechanism isprovided in a first bypass pipe 51. The rotary screw 71 has a rotatingshaft that is parallel to the longitudinal direction of the first bypasspipe 51.

The rotary screw 71 is rotated by a motor 72, and accordingly a mistdeposited in the first bypass pipe 51 can be sent to the upstream sideor the downstream side. This structure prevents fine particles fromdepositing in the first bypass pipe 51. The device for producingfluorine gas in the eighth alternative embodiment has substantially thesame constitution as the device for producing fluorine gas in the secondalternative embodiment except the above structure, and thus similarstructures are not described.

Ninth Alternative Embodiment

A ninth alternative embodiment will be described with reference to FIG.12. A device for producing fluorine gas in the ninth alternativeembodiment differs from that in the second alternative embodimentillustrated in FIG. 5 in the clogging suppression mechanism. In thedevice for producing fluorine gas in the ninth alternative embodiment,an airflow generator 73 constituting the clogging suppression mechanismis provided on a first bypass pipe 51. The airflow generator 73 sends anairflow (for example, a nitrogen gas stream) from the upstream sidetoward the downstream side in the first bypass pipe 51 and increases theflow rate of an anode gas flowing in the first bypass pipe 51. Thisstructure prevents fine particles from depositing in the first bypasspipe 51.

In the embodiment, the flow rate of an anode gas flowing in the firstbypass pipe 51 is preferably 1 m/sec or more and 10 m/sec or less. Theflow rate can be increased to more than 10 m/sec, but in such a case,the pipe resistance in the first bypass pipe 51 increases the pressureloss, and the pressure in an anode chamber 22 of an electrolytic cell 11increases. The pressure in the anode chamber 22 and the pressure in acathode chamber 24 are preferably substantially the same. If thedifference between the pressure in the anode chamber 22 and the pressurein the cathode chamber 24 were excessively large, an anode gas could goover a partition wall 17 and flow into the cathode chamber 24, andfluorine gas could be reacted with hydrogen gas to impair fluorine gasgeneration.

The device for producing fluorine gas in the ninth alternativeembodiment has substantially the same constitution as the device forproducing fluorine gas in the second alternative embodiment except theabove structure, and thus similar structures are not described.

Tenth Alternative Embodiment

A tenth alternative embodiment will be described with reference to FIG.13. In a device for producing fluorine gas in the tenth alternativeembodiment, a first average particle size measurement unit 31 isprovided on an electrolytic cell 11, and the average particle size of amist is measured by introducing the anode gas in the electrolytic cell11 directly into the first average particle size measurement unit 31.The device for producing fluorine gas in the tenth alternativeembodiment has no second average particle size measurement unit 34. Thedevice for producing fluorine gas in the tenth alternative embodimenthas substantially the same constitution as the device for producingfluorine gas in the ninth alternative embodiment illustrated in FIG. 12except the above structure, and thus similar structures are notdescribed.

EXAMPLES

The present invention will next be described more specifically withreference to examples and comparative examples.

Reference Example 1

An electrolyte was electrolyzed to produce fluorine gas. As theelectrolyte, a mixed molten salt (560 L) of 434 kg of hydrogen fluorideand 630 kg of potassium fluoride was used. As the anode, 16 amorphouscarbon electrodes manufactured by SGL Carbon (30 cm in width, 45 cm inlength, and 7 cm in thickness) were placed in an electrolytic cell. Asthe cathode, punching plates formed from Monel (trademark) were placedin the electrolytic cell. One anode faced two cathodes, and portions ofone anode facing the cathodes had a total area of 1,736 cm².

The electrolysis temperature was controlled at 85 to 95° C. First, thetemperature of the electrolyte was set at 85° C., and a direct currentof 1,000 A was applied at a current density of 0.036 A/cm² to startelectrolysis. At the start, the electrolyte had a water concentration of1.0% by mass. The water concentration was measured by Karl Fischeranalysis method.

Electrolysis was started in the above conditions, and for 10 hours fromthe start of the electrolysis, small explosive sound was observed nearthe anodes in the anode chamber. The explosive sound is supposed to becaused by reaction of fluorine gas generated and water in theelectrolyte.

The fluid generated on the anodes at this stage was collected when sentout from the anode chamber of the electrolytic cell to the outside, andthe mist contained in the fluid was analyzed. As a result, 1 L of thefluid generated on the anodes contained 5.0 to 9.0 mg of fine particles(calculated assuming that the mist has a specific gravity of 1.0 g/mL,hereinafter the same is applied), and the fine particles had an averageparticle size of 1.0 to 2.0 μm. The fine particles were observed underan optical microscope, and particles having a hollow spherical shapewere mainly observed. At this stage, the current efficiency of fluorinegas production was 0 to 15%.

The electrolysis continued until the electric energy reached 30 kAh, andthe frequency of explosive sound in the anode chamber was reduced. Atthis stage, the electrolyte had a water concentration of 0.7% by mass.The fluid generated on the anodes at this stage was collected when sentout from the anode chamber of the electrolytic cell to the outside, andthe mist contained in the fluid was analyzed. As a result, 1 L of thefluid generated on the anodes contained 0.4 to 1.0 mg of a mist, and themist had an average particle size of 0.5 to 0.7 μm. At this stage, thecurrent efficiency of fluorine gas production was 15 to 55%. The step ofelectrolysis from the start of electrolysis to this stage is regarded as“step (1)”.

Following the step (1), the electrolyte was continuously electrolyzed.Accordingly, hydrogen fluoride was consumed, and the level of theelectrolyte was reduced. Hence, hydrogen fluoride was appropriatelyresupplied from a hydrogen fluoride tank into the electrolytic cell. Thehydrogen fluoride to be resupplied had a water concentration of 500 ppmby mass or less.

When the electrolysis was continued until the electric energy reached 60kAh, the mist contained in the fluid generated on the anodes had anaverage particle size of 0.36 pm (i.e., 0.4 μm or less). At this stage,absolutely no explosive sound was observed in the anode chamber. At thisstage, the electrolyte had a water concentration of 0.2% by mass (i.e.,0.3% by mass or less). At this stage, the current efficiency of fluorinegas production was 65%. The step of electrolysis from the end of thestep (1) to this stage is regarded as “step (2)”.

Following the step (2), the current was increased to 3,500 A to increasethe current density to 0.126 A/cm², and the electrolyte was continuouslyelectrolyzed. The fluid generated on the anodes at this stage wascollected when sent out from the anode chamber of the electrolytic cellto the outside, and the mist contained in the fluid was analyzed. As aresult, 1 L of the fluid generated on the anodes contained 0.03 to 0.06mg of fine particles, and the fine particles had an average particlesize of about 0.2 μm (0.15 to 0.25 μm) with a particle size distributionof about 0.1 to 0.5 μm. FIG. 14 illustrates the measurement result ofparticle size distribution of the fine particles. At this stage, thecurrent efficiency of fluorine gas production was 94%. At this stage,the electrolyte had a water concentration of less than 0.2% by mass. Thestep of electrolysis from the end of the step (2) to this stage isregarded as “stable step”.

Details of the electrolysis performed as above in Reference Example 1are summarized in Table 1. Table 1 illustrates electric current,electrolysis time, electric energy, the water concentration in anelectrolyte, the mass of a mist contained in 1 L of a fluid generated onthe anodes (“anode gas” in Table 1) , the average particle size of amist, current efficiency, the amount of a fluid (containing fluorinegas, oxygen gas, and a mist) generated on the anodes, the amount of amist generated on the anodes, the intensity of explosive sound, and thewater concentration in a fluid formed on the cathodes (the waterconcentration in a cathode gas” in Table 1).

A graph representing the relation between average particle size of amist and amount of the mist generated on the anodes is illustrated inFIG. 15. The graph in FIG. 15 reveals that the average particle size ofa mist has a relation to the amount of a mist generated on the anodes.As the amount of a mist generated increases, the clogging of pipes andvalves is more frequently caused. When a mist having an average particlesize of more than 0.4 μm is generated, the amount of a mist generatedincreases, and the mist is settled by gravity. The relation representedby the graph in FIG. 15 therefore illustrates a relation between theaverage particle size of a mist and likelihood of clogging of pipes andvalves.

A graph representing the relation between average particle size of amist and water concentration in an electrolyte is illustrated in FIG.16. As the average particle size of a mist increases, the clogging ofpipes and valves is more frequently caused. The relation represented bythe graph in FIG. 16 therefore illustrates a relation between the waterconcentration in an electrolyte and likelihood of clogging of pipes andvalves.

TABLE 1 Electrolysis Water Mist in anode gas Intensity of Water Electricconcentration Average Current explosive concentration Electric Elapsedenergy in electrolyte Amount particle efficiency sound in cathode gasStep current (A) time (h) (kAh) (% by mass) (mg/L) size (mm) (%) (dB) (%by volume) Step (1) 1000  0-30  0-30 1.0 5.0-9.0 1.0-2.0  0-15 50-700.10 Step (1) 1000 30 30   0.7 0.4-1.0 0.5-0.7 15-55 25-35 0.07 Step (2)1000 60 60   0.2 not measured  0.36 65 15-30 0.02 Stable step 3500 6577.5  less than 0.2 0.03-0.06 0.15-0.25 94 2-5 not measured

Example 1

Electrolysis was performed in the same manner as in Reference Example 1using the device for producing fluorine gas illustrated in FIG. 2. Inthe electrolysis in the step (1), the fluid generated on the anodes wasallowed to flow through the second bypass pipes, the bypass valves, andthe first bypass pipe. After the completion of electrolysis in the step(1), the electrolysis was once stopped, and the inside of the device forproducing fluorine gas was inspected. As a result, a mist deposited inthe first bypass pipe, but the first bypass pipe had a large pipediameter, and thus the pipe was not clogged.

The electrolysis reached the step (2) where the mist had an averageparticle size of 0.4 μm or less (the electrolyte had a waterconcentration of 0.2% by mass that was not more than the referencevalue, 0.3% by mass), and thus the fluid generated on the anodes wasallowed to flow through the first pipes, the first pipe valves, thefourth pipe, and the first mist removal unit. Neither mist depositionnor clogging was caused in the first pipes, the first pipe valves, orthe fourth pipe, but the fluid generated on the anodes was fed to thefirst mist removal unit, and the mist was removed by the first mistremoval unit. The first mist removal unit was a scrubber type mistremover that sprayed liquid hydrogen fluoride to remove microparticlessuch as a mist and had a mist removal rate of 98% or more.

Comparative Example 1

Electrolysis was performed in the same manner as in Example 1 exceptthat the fluid generated on the anodes in the electrolysis in the step(1) was allowed to flow through the first pipes, the first pipe valves,the fourth pipe, and the first mist removal unit.

During the electrolysis in the step (1), of pressure gauges attached tothe anode side and the cathode side of the electrolytic cell, themeasured value of the pressure gauge at the anode side graduallyincreased, and the differential pressure from the pressure at thecathode side reached 90 mmH₂O. The electrolysis was thus stopped. Thereason for the stop is described below. Of the partition wall in theelectrolytic cell, a portion immersed in the electrolyte had a verticallength (immersion depth) of 5 cm. If the pressure at the anode side werehigher than the pressure at the cathode side by about 100 mmH₂O, thesurface of the electrolyte at the anode side would be below the lowerend of the partition wall. As a result, fluorine gas would flow over thepartition wall and be mixed with hydrogen gas at the cathode side tosuddenly cause dangerous reaction between fluorine gas and hydrogen gas.

After the system was purged with nitrogen gas or the like, the insidesof the first pipes, the first pipe valves, and the fourth pipe wereinspected. The first pipes were not clogged because the pipes extendedin the vertical direction. Deposition of a small amount of fineparticles was observed in the first pipe valves, and the inlet portionsto the downstream pipe of the first pipe valves, or to the fourth pipe,were clogged with fine particles. Deposition of fine particles was alsoobserved in the fourth pipe, but the deposition was such a small amountas not to clog the pipe.

REFERENCE SIGNS LIST

1 sample chamber

2 light source

3 scattered light detection unit

4A, 4B transparent window

10 electrolyte

11 electrolytic cell

13 anode

15 cathode

22 anode chamber

24 cathode chamber

31 first average particle size measurement unit

32 first mist removal unit

33 second mist removal unit

34 second average particle size measurement unit

36 water concentration measurement unit

41 first pipe

42 second pipe

43 third pipe

44 fourth pipe

45 fifth pipe

46 sixth pipe

47 seventh pipe

48 eighth pipe

49 ninth pipe

51 first bypass pipe

52 second bypass pipe

61 first pipe valve

62 bypass valve

F fluid

L light for light scattering measurement

M mist

S scattered light

1. A method for producing fluorine gas, the fluorine gas being producedby electrolyzing an electrolyte containing hydrogen fluoride and a metalfluoride, the method comprising: electrolyzing the electrolyte in anelectrolytic cell; measuring a water concentration in the electrolyte inthe electrolyzing; and sending a fluid generated in an inside of theelectrolytic cell in the electrolyzing the electrolyte, from the insideto an outside of the electrolytic cell through a flow path, wherein inthe sending, the flow path in which the fluid flows is switched inaccordance with the water concentration in the electrolyte measured inthe measuring a water concentration, such that the fluid is sent to afirst flow path that sends the fluid from the inside of the electrolyticcell to a first outside when the water concentration in the electrolytemeasured in the measuring a water concentration is not more than apredetermined reference value, or the fluid is sent to a second flowpath that sends the fluid from the inside of the electrolytic cell to asecond outside when the water concentration is more than thepredetermined reference value, and the predetermined reference value isa numerical value of 0.1% by mass or more and 0.8% by mass or less. 2.The method for producing fluorine gas according to claim 1, wherein themetal fluoride is a fluoride of at least one metal selected from thegroup consisting of potassium, cesium, rubidium, and lithium.
 3. Themethod for producing fluorine gas according to claim 1, wherein an anodeused in the electrolyzing is a carbonaceous electrode formed from atleast one carbon material selected from the group consisting of diamond,diamond-like carbon, amorphous carbon, graphite, and glassy carbon. 4.The method for producing fluorine gas according to claim 1, wherein theelectrolytic cell has a structure in which bubbles generated on an anodeor a cathode used in the electrolyzing are capable of rising verticallyin the electrolyte to reach a surface of the electrolyte.
 5. A devicefor producing fluorine gas, the fluorine gas being produced byelectrolysis of an electrolyte containing hydrogen fluoride and a metalfluoride, the device comprising: an electrolytic cell storing theelectrolyte and configured to perform the electrolysis; a waterconcentration measurement unit configured to measure a waterconcentration in the electrolyte in the electrolytic cell during theelectrolysis; and a flow path configured to send a fluid generated in aninside of the electrolytic cell during the electrolysis of theelectrolyte, from the inside to an outside of the electrolytic cell,wherein the flow path includes a first flow path configured to send thefluid from the inside of the electrolytic cell to a first outside and asecond flow path configured to send the fluid from the inside of theelectrolytic cell to a second outside and includes a flow path switchingunit configured to switch the flow path in which the fluid flows, to thefirst flow path or the second flow path in accordance with the waterconcentration in the electrolyte measured by the water concentrationmeasurement unit, the flow path switching unit is configured to send thefluid from the inside of the electrolytic cell to the first flow pathwhen the water concentration in the electrolyte measured by the waterconcentration measurement unit is not more than a predeterminedreference value, or to send the fluid from the inside of theelectrolytic cell to the second flow path when the water concentrationis more than the predetermined reference value, and the predeterminedreference value is a numerical value of 0.1% by mass or more and 0.8% bymass or less.
 6. The method for producing fluorine gas according toclaim 2, wherein an anode used in the electrolyzing is a carbonaceouselectrode formed from at least one carbon material selected from thegroup consisting of diamond, diamond-like carbon, amorphous carbon,graphite, and glassy carbon.
 7. The method for producing fluorine gasaccording to claim 2, wherein the electrolytic cell has a structure inwhich bubbles generated on an anode or a cathode used in theelectrolyzing are capable of rising vertically in the electrolyte toreach a surface of the electrolyte.
 8. The method for producing fluorinegas according to claim 3, wherein the electrolytic cell has a structurein which bubbles generated on the anode or a cathode used in theelectrolyzing are capable of rising vertically in the electrolyte toreach a surface of the electrolyte.